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Very
good questions. Thanks for that. The GBS Report explains in huge and intimate
detail everything you could possibly want to know about the sample size, the
methods, the history, the biases and limitations. Although I often post extracts
of individual species texts to this chat line to answer particular questions
about a particular species, there is no way I am going to be posting whole slabs
of the introductory section on a chat line. Read it! (Canberra Birds: A
Report on the first 21 years of the Garden Bird Survey). If you can find
and demonstrate any inadequacies in these aspects of The GBS Report (and
after nearly 10 years no one yet has), it would be nice to know. And yes the
results do have limitations and these are fully understood and explained
therein. Show me a scientific report on natural populations that doesn't have
limitations.
You
refer to scientific data and analyses on Southern
Bluefin Tuna (SBT) population coming from significant
differences in the two countries' scientists' methodologies in estimating
and I suggest no doubt differences in what they
wished to prove. But the GBS is one system in one location. And yes the methods
changed slightly during the course of the survey but this is explained in
extreme detail in The GBS Report.
Maybe
you are getting to the huge issue about whether the Canberra and region suburbia
is representative of the wider SE Australian environment. Very good question.
The GBS Report mentions but does not dwell on that issue, as it does not
carry extra-regional data to make the comparison.
Yes as
these are both common, widespread, easy to find and identify species,
unlikely to be overlooked or misidentified, more than to a small amount
that would be fairly constant over the years. I am confident that the whistler (both species) sample areas used in
the data/analysis that have are now being presented and discussed are
"representative" enough, and the number of sightings
made (whether positive or not) are large enough to
enable reliable inferences to be drawn. Even if on
some questions the inferences are that there is probably no change or not yet
enough data to show a trend. More to the point, that is the data that we have.
There is no other data against which to measure validate or compare these
things. What is the alternative? People describe their individual vague
impressions and we take that as a baseline and then say oh the survey data of
857483 individual count observations (over the first 21 years) on 53244 observer
weeks and 1316 observer years (all these figures are much higher now) is less
accurate than impressions!
Don't
read Birds in Canberra Gardens for this. It is crap, crass, inadequate and
deceptive in most respects of explaining these things.
Can
people please learn the difference between the word "sites" (places) as in "actual bird sitings" which my
spell check doesn't approve of and if it is a word would mean the actual place
where some bird was and sights & sightings
(observations).
Philip
At risk of being (pretty rightly) accused of
getting involved in something about which, in detail, I know very little (I'm a
very amateur bird-watcher, though I do have qualifications in science and
economics of some relevance to the question I'm asking!), I would be interested
to know something about the statistical "reliability" of the data shown in the
last few days on rufus and golden whistler population time series trends and
population seasonalities. My sense is that only if the data are
statistically sound can reasonably reliable general conclusions be inferred from
them. (I'm not for one moment suggesting there is
a "reliability problem" - it's just that I've not seen any comment on the
question, either way).
I am wondering (a) what sample sizes are
these data based on - essentially how many actual bird sitings and how many
"nil" sitings from how many observation sites/areas are represented by each data
point on the graphs; and (b) what proportion of the "area/region of
interest" (eg, the ACT if it's trends in the ACT that are being
reported) - or perhaps more usefully what proportion of the species'
typical habitat' in (say) the ACT, was included in the aggregate
data?
I ask these questions, in part due to prior
experience in ostensibly scientific data and analyses on Southern Bluefin Tuna
(SBT) population, when I was involved as a policy adviser in trying to interpret
the analyses from what had turned into a major dispute between Australian
and Japanese fish population scientists over SBT populations and
"responsible" fishing quotas.
To simplify the detail of the analysis, the dispute
turned on significant differences in the two countries' scientists'
methodologies in estimating (and projecting) current and future SBT
populations. In part, an element in the different analyses and conclusions
was the extent to which population data from a set of "sample areas" which
necessarily covered only a small proportion of the total range of SBT could, or
could not, be regarded as "representative".
So, to put my questions that way, can you be
confident that the whistler (both species) sample areas used in the
data/analysis that have are now being presented and discussed are
"representative" enough , and the number of sitings made (whether positive or
not) large enough to enable reliable inferences to be drawn?
Thank you.
Kevin Bray
02
6251 2087
0406 376 878
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